The use of contact terminals as connection means for cable ends is well known in the art. Commonly, cables are fixed to contact terminals by crimping a stripped cable strands to a connection portion of the contact terminal. However, many applications demand for more reliable electrical contact and/or reduced contact resistance between cable and terminal. This is particularly important in the case where cables and contact terminals of different material have to be connected. Here, due to different physical properties of the members, connections that first appear to be reliable may deteriorate with time.
A typical example for the latter case is the use of aluminum cables together with copper contact terminals. The terms aluminum and copper as used herein, denote the commonly used forms of these materials as cable or terminal material, including e.g. also aluminum and copper alloys, which are alloys having a fraction of at least 50% of aluminum or copper, respectively. The use of aluminum cables for example in automotive applications gains more and more interest since aluminum is lighter and less expensive as compared to the commonly used copper. Although an aluminum cable needs to be of larger cross section (about a factor of 1.6) to transmit the same current as a copper cable, still, by replacing the copper cable with an aluminum cable, a significant weight reduction (e.g. 10-20 kg per car) can be achieved. Thus, aluminum is very attractive for the use as wire material; however, the use of aluminum cables poses new problems which do not occur in the case of using copper cables.
Since still a large number of electrical components as e.g. contact terminals have to be made from copper, aluminum cables have to be connected to copper contact terminals. If e.g. an aluminum cable is crimped to a copper contact terminal, the crimped aluminum cable end tends to “flow”, i.e. the connection can become loose. Further, aluminum tends to corrode forming a layer of aluminum oxide which has poor electrical conductivity. This is in particular a problem if aluminum and another metal are in contact and an electrolyte (e.g. moisture) is present in between the contact faces. In this case, the two different metals facilitate corrosion. It reduces the reliability and the durability of e.g. a crimp connection between an aluminum cable and a copper contact terminal significantly.
Thus, connecting aluminum cables to other electrical means, such as e.g. contact terminals, demands for new solutions, in particular if said electrical means are made from a different material.
A document dealing with this problem is DE 197 27 314 A1, which discloses a method to secure the crimping of cables to copper contact terminals by applying a conductive glue to the crimping zone. The glue serves to reduce the contact corrosion of the two members to be crimped. However, the use of glue is not always possible and/or satisfying.
A further method to secure a crimp connection between an aluminum cable and e.g. a contact terminal is described in EP 1 817 819 B1. In this document, it is suggested to insert a contacting material, e.g. tin or a tin alloy, between the crimping portion of the contact terminal and the aluminum cable. Said contacting material is heated up by means of a flame or a laser until it melts. Then, upon solidification, it forms a material connection between the cable and the contact terminal. During the heating or after, the contact terminal and the cable are crimped together.
Document EP 1 730 813 B1 discloses yet another method to improve the connection of a copper contact terminal to an aluminum cable. This document suggests establishing a permanent connection between the two members by means of plasma welding. To establish the electrical connection the contact terminal is coated with nickel while the cable is coated with zinc and the two coatings are mutually connected by plasma welding. However, this technique is rather complex because it involves a plasma welding process which is not easy to implement in particular on large connection areas.
An aim of the invention is to provide an electrical contact terminal which comprises an improved connection portion for the mounting of an electrical cable. The term “cable” as used herein is to be understood in its broadest meaning and denotes any kind of conductor commonly attached to contact terminals, as e.g. wires (coated and uncoated), strands, leads, cable sheathing, etc. It is a further object to the present invention to provide a new method of manufacturing said contact terminal.